Method for completing wells to prevent paraffin deposits



July 22, 1969 L.. A. MCDOUGALL ETAL METHOD FOR COMPLETING WELLS TO PREVENT -PARAFFIN DEPOSITS Filed Feb. l, 1967 FIG'. 2

FRED A. BROOKS, JR.

LEE A. MCDOUGALL INVENTORS BYWJE'QHL ATTORNEY United States Patent O 3,456,735- METHOD FOR COMPLETlNG WELLS TO PREVENT PARAFFIN DEPOSITSV, Lee A. McDougall, Bellaire,l and Fred A. Brooks, Jr., Houston, Tex., assignors to Esso Production Research Company, a corporation of Delaware Filed Feb. 1, 1967, Ser. No. 613,240

Int. Cl. E21b.43/00 U.S. Cl. 166-304 5 Claims ABSTRACT oF THE DISCLOSURE t l A cellular insulating material is installedaround the producing string'in an oil well to prevent the'deposition of paraflin from crude oil passing through the string to the earths surface.

` BACKGROUND oF THE iNvENTIoN v Field of the invention This invention relates to the completion of oil and gas wells and is particularly concerned with a method for completing wells producing waxy crude oils to prevent the formation of paratln deposits.

Description of the prior art The formation of paraflin deposits is a serious problem in many wells producing heavy crude oils. Such oils often contain long chain parainic hydrocarbons, asphaltenes,l and resins which are relatively soluble at reservoir ternperatures but relatively insoluble at slightly lower temperatures. The cooling which takes place as such oils move upwardly in the wellbore may be sufficient to precipitate these materials and hence parain deposits may SUMMARY OF THE INVENTION It has now been found that paraiin deposits can often be preventedl by installing a cellular insulating material around the producing strings. Studies have shownA that the heat losses through the pipeV walls in most wells are relatively high and that the oil temperature drops rapidly as uids move upwardly through the tubing or casing to the earths surface. By installing a rigid foam or similar porous material in the annulus surrounding theproducing string, the geothermal energy present in the oil can often be conserved so that the oil temperature does not drop to the point where parain deposition takes place. This avoids the necessity for periodically treating the well to remove deposits and eliminates many other problems associated with parain deposit formation.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 in the drawing is a schematic representation of a well provided with a rigid foam in the annulus between the tubing string and casing 'and FIGURE 2 depicts the use of insulating material above the cement in a multiple completion well.

3,456,735 Patented July 22, 1969 ICC ` DESCRIPTION OF THE PREFERRED EMBODIMENTS f A variety of unicellular plastic foams which are rigid and relatively inert to water, brine and othersubterranean fluids under reservoir conditions may be employed in` carrying out the invention. Suitable materials vinclude the expanded phenol-aldehyde and urea-aldehyde resins, polystyrenes, polyurethanes, polyethylenes, cellulose acetates and the like. These materials have very low thermal conductvities, generally on vthe order of about 0.02 to 0.03 B.t.u./hr./ft.2/F./ft., and aretherefore excellent insulating materials. lIn general their heat resistance, chemical stability and mechanical strength are sufficient to `permit their use under most conditions ylikely to be encountered in producing wells. Certain of the expanded materials have better properties than others and are therefore preferred.' Y

A particularly eiective class of expanded materials for use in wells where moderate temperature conditions prevail are the unicellular polyurethanes. These may be prepared by the reaction of a dicarboxylic acid ester of a polyfunctional alcohol containing free hydroxyl groups, an adipic :acid ester of glycerol for example, vwith a dior polyisocyanate such as toluene di-isocyanate. The dior polyisocyanate reacts in part with the hydroxyl groups on the ester to form polyurethane bonds. Due to the increase in temperature as the exothermic reaction takes place, the carboxyl groups react with isocyanate groups to form cross-links and liberate carbon dioxide. A tough spongy mass which turns into a very hard rigid foam is formed. A thermal blowing agent such as trichloromonouoromethane, Unicel or Porofor may be included to increase the production of gas and thus obtain a lower density material. Surface active agents, fillers and other additives can also be included if desired. This method and other techniques which may be used to produce rigid polyurethane foams, including the use of free flowing powders which melt, expand and cure when heated, have been described in detail in the literature and will be familiar to those skilled in theart. Prepared compositions based on these techniques which can be used to produce the rigid foams are available commercially.

Small spheres or microballoons of phenolic plastic or similar material which ca-n be bonded together with a polyester, phenolic or epoxy binder may also be used to produce cellular materials within the wellbore. The bonding action produces a rigid, low density material sometimes referred to as a syntactic foam. The spheres used are generally filled with nitrogen and normally have diameters on the order from about 0.001 to about 0.0015` inch. The use of such materials is particularly advantageous in that the bonding does not require heat and can be carried out insitu without difficulty.

The cellular plastics referred to above arey utilized n accordance with the invention to form zones of low thermal conductivity surrounding the tubing or casing strings in wells which produce crude oils normally presenting paran deposition problems. The location'of the low thermal conductivity zones will depend primarily upon the way in which the well is completed. FIGURE l of the drawing depicts a well producing oil from a subterranean formation through a single string of tubing 11. Surface pipe 12 surrounded by cement 13 extends through the shallow formations near the earths surface. Casing string 14 extends from the surface to a point below perforations 15 and is surrounded by cement sheath 16. A production packer 17 has been installed in the well between the tubing Iand casing afshort distance above the producing zone. A packer uid 18 is located in the annulus abveithefpackeir Vto equalize the pressure across itu-and A'This material is located immediately above the packer fluid and extends upwardly to the earths surface. The location rof thev bottom 'of the plastic is determined prima'fr'ily by the heat losses which take place as the produced 4 oil flows upwardly through the tubing string'. The plastic should' extend downwardly far enough to prevent cooling ofthe oil below the cloud point before it reaches the earths surface. In most cases the lower end of the plastic should extend from the surface to a depth of about 1000 to about 2000 feet. The geothermal gradient for the particular area, ythe depth of the formation, the expected production rate of the well, the water-oil ratio, the cloud point of the particular oil and other considerations may dictate location of the plastic at a` shallower or w deeper point. Heat transfer calculations based on temperature surveys can be used to determine the optimum location for the plastic material. As pointed out earlier, the weight of the packer uid may be increased to maintain the required hydrostatic head if the height of the iiuid column is less than that which would otherwise be used.

FIGURE 2 of the drawing depicts a tubingless multiple completion well in which a unicellular plastic foam has been installed to prevent paraffin deposition. The well shown contains pipe strings 20 and 21 which extend through surface pipe 22 and the surrounding cement sheath 23 to the producing zones. The lower section of the well has been cemented in the usual manner, as indicated -by reference numeral 24, and the producing strings have been perforated to provide communication with the oil bearing formations. The perforations in the upper zone are indicated by reference numeral 25; while those in the lower zone are indicated by reference numeral 26. Conventional cementing shoes 27 and 28 are located at the lower ends of the pipe strings. Cellular plastic material 29 iills the space surrounding the producing strings above the cement. This insulates the pipe strings from one another and froml the surrounding formation. Again, the optimum depth to which the cellular plastic should extend below the earths surface `can be readily determined by heat transfer calculations which will be familiar to those skilled in the art.

Y The method utilized for installation ofthe cellular plastic material will depend upon the particular plastic composition selected. Many of the expanded or foamed materials can be vprepared by incorporating Vblowing agents into liquid resin mixtures which on heating liberate gases byv chemical reaction or by adding'compounds to liquid polyesters to form gas and at the `same time cross-link the resulting foam into a rigid structure. These reactions generally require the mixing of two prepared components and the application of heat and can be carried outv by means of electrical heaters lowered into the wellbore through one or more tubing strings. Single compositions which can be injected and later heated in the wellbore to melt, expand and cure them are also available. Volatile carrier uids can be employed to aid in the injection of solid materials. If preformed hollow plastic spheres orfmicroballoons and a binder are used, the application of heat is generally unnecessary. Certain polymers containing blowing agents can also be hardened and solidified to produce rigid foams byheating and subjecting them to radiation from a radioactive SQurce 4 lowered into the wellbore through the producing string. With polyethylene 4and similar polymers, the cross-linking`V reaction takes place very rapidly in the presence of neutrons and gamma rays and results in a marked improvement in the physical properties of the material.

*In carrying out the invention, it is generally necessary to remove the 'packer 'iluidor other liquids from the section of thewwellbo're in which the cellular material is to be placed. Thiscan be done'by lowering a string` of small diameter tubing into the space to be filled with the cellular material" and then injecting air or gas into the space to displace )the liquid tothe surface through the smalldiameter string. After the space has been voided, the material or materials to be used for producing the cellular foam may be introducedl VIf polystyrene beads containing Va blowing agent,v a powdered mixture of a polyurethane type resin containing free isocyanate and ainely divided solid which releases water at .elevated temperatures, or a similar single component system is employed, "the reactive Vmaterial may be introduced through the line used for the removal of fluids from the wellbore. Where two or more reactants must be introduced separately and mixed to form the expanded material in situ, one or more additional strings of small diameter tubing may be provided. In some cases liquid constituents may be preheated at the surface to avoid the necessity ofproviding heat at'the reaction site. ln most cases, however, an electrical heater positioned in the production string about -which the cellular material is to be formed will be utilized to provide the heat needed. Where considerable heat is required, the foam-forming reaction can be carried out in stages, raising the heater inthe tubing string-following the production of Vfoam at each level. In such cases, sucient time for the curing of each batch should be provided` before additional reactive material is introduced into the wellbore. The foamforming reactions generally take place rapidly and'hence the entire operation can be completed in a relatively short time. v

The small dia-meter tubing strings employed for introduction of the reactive material are normally removed from the wellbore before completion of the reaction. The presence of the foam does not interfere with conventional production` and workover operations. In the event, however, that vthe foam must be removed to permit replace'- ment of casing or tubing or for other reasons, most of the foams canbe destroyed by treatment 'with suitable solvents, acidsor bases. Polystyrene, for example, can fbe readily dissolved in aromatic hydrocarbons. Phenol formaldehyde, urea formaldehyde' and7 other phenolic resins decomposein the presence of strongalkalines. Acids can be used for removal of the polyurethanes. The method ,Of the invention is particularly advantageous for overcoming paratlin deposition problems in offshore. Wells where workover equipment lisnot readily available. The foamed materials can be readily installed in the annular spaces ofwells completed in a variety of different ways andar'erelatively inexpensive. They provide highly ef-` fectivel insulation against heat transfer and thus permit the control. of parain deposition at relatively low costs.

We claim: v

1. A method for preventingthe formation of parain deposits in awellcontaining a pipe string through which waxy crude oil is withdrawn to the earths surface without heating the oilin place which comprises installing a rigid cellular plastic foam in said well, said foam surrounding said" pipe string and extendingv into the well from a point near the earths surface to a depth suicient to` prevent cooling of said oil to a temperature :be-

low the cloud point as the oil passesthrough said pipe' string.

2. A method as defined by claim 1 wherein said-plastic` foam is a unicellular polyurethane,

3. A method as dened by claim 1 wherein said rigid References Cited foam is produced in situ by heating foam-producing re- UNITED STATES PATENTS actants within the wellbore.

4. A method as defined by claim 1 wherein said rigid 895,612 8/1908 Baker 166-57 foam is produced by the bonding together of gas filled 5 2,245,870 6/1941 Norman 166-41 plastic Spheres 3,106,227 10/ 1963 Crowley 13S-149 tic in the wellbore in the presence of radiation. JAMES A. LEPPINK, Primary Examiner 

